1. Field of the Invention
The present invention relates generally to a wireless local area network (WLAN) system, and in particular, to a scheduling method for guaranteeing the reliability of voice packet transmission in a WLAN system.
2. Description of the Related Art
Given the rapid spread of WLAN systems, ongoing research for supporting real time services, such as a voice call, which is very sensitive to delay, is underway. A rapid growth of a wireless network market has resulted in the implementation of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard for WLAN systems. in 1997. The IEEE 802.11 standard describes the specifications of a media access control (MAC) layer and a physical (PHY) layer for WLAN systems. IEEE 802.11 WLAN operates using dual channel access mechanisms, i.e., a distributed coordination function (DCF) mode and a point coordination function (PCF) mode. The DCF mode is based on a carrier sense multiple access with collision avoidance (CSMA/CA) channel access mechanism, while the PCF is based on a polling mechanism.
A DCF mode operates in a distributed manner and is a basic channel access mechanism of WLAN. However, it is difficult to guarantee quality of service (QoS) in the DCF mode. Thus, a significant problem can occur when a WLAN must transmit real-time traffic. In general, a typical WLAN system having an 11 Mbps bandwidth can support Voice over Internet Protocol (VoIP) connection very restrictively in the DCF mode. For this reason, the DCF is a mechanism for which it is difficult to satisfy QoS required in the real-time traffic.
A PCF mode is an alternative for transmitting real-time traffic in a IEEE 802.11 WLAN system and is a suitable mechanism for supporting a voice service based on a central control.
FIG. 1 is a conceptual diagram for explaining DCF and PCF mode operations as defined in the IEEE 802.11 standard.
In the PCF mode, a point coordinator (PC) is required as a central control office. Although any node in a network can be the PC, in a system set as an infrastructure network including an access point (AP), the AP generally plays a role of the PC.
A period in which the system operates in the PCF mode is a contention free period (CFP), while a period in which the system operates in the DCF mode is a contention period (CP). A super frame includes the CFP and the CP. Each CFP begins from beacon frame transmission, and the CFP and the CP are alternates. At a predetermined time, the PC generates a beacon frame B, which is defined by a CFP repetition interval (CFPRI) parameter and determines a target beacon transmission time (TBTT). The length of the CFP is determinable by the size of an available traffic and polling list. The PC can terminate a CFP during or before the maximum duration (CFPMaxDuration).
However, since a polling list scheduling mechanism has not been defined in detail in the PCF mode for the IEEE 802.11 standard, research concerning the polling list scheduling mechanism is ongoing.
A conventional round robin mechanism can be considered for the PCF mode scheduling. However, the round robin mechanism is ineffective since voice and data traffic is processed in the same manner, and in particular, a round robin algorithm generates a long packet delay and has a high dropping probability in a high-load environment.
To address this processing delay problem, various round-robin-based scheduling mechanisms have been suggested. However, no suggested mechanism satisfy the improvement of the packet delay and dropping probability, which are disadvantages of the round robin mechanism, to be applied to real-time traffic transmission in WLAN.